Image forming apparatus correcting magnification of image in scanning direction of light beam

ABSTRACT

An image forming apparatus including: a light source configured to emit a light beam based on an image signal; a deflector configured to deflect the light beam so that the light beam emitted from the light source is scanned on a surface of a photosensitive member in a main scanning direction; a storage portion configured to store a first magnification of an image with respect to a scanning position in the main scanning direction; and a controller configured to generate a second magnification of the image with respect to a reference color image, wherein the controller generates a third magnification based on the first magnification corrected based on the second magnification and the second magnification to correct the image signal based on the third magnification.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an image forming apparatus including adeflector configured to deflect a light beam.

Description of the Related Art

An electrophotographic image forming apparatus is configured to deflecta light beam emitted from a light source based on an image signal with arotary polygon mirror, and cause the deflected light beam to form animage as a light spot scanned on a photosensitive member at a constantspeed by an fθ lens. However, a scanning position of the light spot bythe fθ lens may be positionally displaced by a part tolerance to distortan output image. To address this problem, in Japanese Patent ApplicationLaid-Open No. 2009-17396, at a time when an image forming apparatus isassembled, a magnification of an image with respect to a scanningposition is measured, a measurement result is stored in advance as aprofile magnification, and an image signal is corrected based on theprofile magnification.

Meanwhile, in an image forming apparatus configured to overlap aplurality of colors with one another to form a color image, amagnification of an image of each color may be changed by a parttolerance, a temperature in use, and a change with time to cause colormisregistration among the images of the plurality of colors overlappedwith one another. To address this problem, in Japanese PatentApplication Laid-Open No. H07-52468, a color misregistration measurementpattern for measuring the color misregistration is formed, and a colormisregistration magnification of an image of each color is correctedbased on a detection result of the color misregistration measurementpattern.

However, when the magnification of the image with respect to thescanning position and the magnification for correcting the colormisregistration are corrected in combination, a position at which themagnification of the image with respect to the scanning position iscorrected may be displaced in accordance with the magnification forcorrecting the color misregistration. The displacement of the correctedposition has a problem in that a density variation or moire is caused inthe images of the plurality of colors overlapped with one another.

SUMMARY OF THE INVENTION

In view of the above-mentioned circumstances, the present inventionprovides an image forming apparatus capable of reducing colormisregistration at a time when a first magnification and a secondmagnification of an image in a main scanning direction are corrected incombination.

According to one embodiment of the present invention, there is providedan image forming apparatus comprising:

a light source configured to emit a light beam based on an image signal;

a deflector configured to deflect the light beam so that the light beamemitted from the light source is scanned on a surface of aphotosensitive member in a main scanning direction;

a storage portion configured to store a first magnification of an imagewith respect to a scanning position in the main scanning direction; and

a controller configured to generate a second magnification of the imagewith respect to a reference color image,

wherein the controller generates a third magnification based on thefirst magnification corrected based on the second magnification and thesecond magnification to correct the image signal based on the thirdmagnification.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an image forming apparatus.

FIG. 2 is a plan view for schematically illustrating components that arearranged inside a light scanning apparatus.

FIG. 3 is a flowchart for illustrating magnification correctionoperation on an image in a main scanning direction.

FIG. 4A and FIG. 4B are an explanatory view and an explanatory diagramof a profile measuring device, respectively.

FIG. 5 is a graph for showing a displacement amount with respect to ascanning position.

FIG. 6 is a graph for showing a profile magnification with respect tothe scanning position.

FIG. 7 is a diagram for illustrating color misregistration measurementpatterns and pattern detectors.

FIG. 8 is a graph for showing a displacement amount with respect to thescanning position.

FIG. 9 is a graph for showing a profile magnification and a compositemagnification with respect to the scanning position.

FIG. 10 is an enlarged graph for showing an end of the profilemagnification and the composite magnification with respect to thescanning position.

FIG. 11 is a graph for showing a profile magnification and a compositemagnification with respect to the scanning position in a case of no fθlens.

FIG. 12 is an enlarged graph of an end of the profile magnification andthe composite magnification with respect to the scanning position in thecase of no fθ lens.

FIG. 13 is a flowchart for illustrating magnification correctionoperation on an image in the main scanning direction in the case of nofθ lens.

FIG. 14 is a graph for showing a corrected displacement amount withrespect to the scanning position.

FIG. 15 is a graph for showing a profile magnification and a correctedcomposite magnification with respect to the scanning position.

FIG. 16 is an enlarged graph of an end of the profile magnification andthe corrected composite magnification with respect to the scanningposition.

FIG. 17A, FIG. 17B, and FIG. 17C are diagrams for illustratingrelationships between an output image and a corrected pixel position.

FIG. 18 is a flowchart for illustrating image forming control operation.

FIG. 19 is a block diagram of an image controller.

FIG. 20A, FIG. 20B, and FIG. 20C are diagrams for illustrating an imagesignal and a pixel size.

FIG. 21 is a diagram for illustrating magnifications and pixel sizes incontinuous output operation of image signals.

FIG. 22 is a flowchart for illustrating image forming control operationbased on displacement between corrected pixel positions.

FIG. 23 is a block diagram of an image controller based on displacementbetween corrected pixel positions.

FIG. 24 is a diagram for illustrating magnifications and pixel sizes incontinuous output operation of image signals.

DESCRIPTION OF THE EMBODIMENTS

Modes for carrying out the present invention are described below withreference to the accompanying drawings.

First Embodiment

(Image Forming Apparatus)

FIG. 1 is a cross-sectional view of an image forming apparatus 100. Theimage forming apparatus 100 is an electrophotographic digital colorprinter that uses toners (developers) of a plurality of colors to form acolor image on a recording medium (hereinafter referred to as “sheet”).The image forming apparatus 100 includes an image reading unit 700configured to read an image of an original, and an image forming unit701 configured to form the image on a sheet.

((Image Reading Unit))

The image reading unit 700 includes an original glass plate 702, anillumination device 703, reflecting mirrors 704 a, 704 b, and 704 c, animaging lens 705, and an image sensor 706 (e.g., CCD) formed of aplurality of light receivers. The illumination device 703 illuminatesthe original placed on the original glass plate 702. Light reflected bythe original is reflected by the reflecting mirrors 704 a, 704 b, and704 c to be guided to the imaging lens 705. The imaging lens 705 imagesthe reflected light on the image sensor 706. The image sensor 706 is aphotoelectric conversion element. The image sensor 706 separates colorsof the reflected light from the original to convert image information ofa blue component (B), image information of a green component (G), andimage information of a red component (R) into electrical image data 30.The image data 30 output from the image sensor 706 is input to an imagecontroller 20 included in the image forming unit 701. The imagecontroller 20 serving as a controller includes a CPU 21, a memory 22,and a color conversion processing portion 23 (FIG. 2). The colorconversion processing portion 23 performs color conversion processingbased on an intensity level of each of an image signal of the bluecomponent (B), an image signal of the green component (G), and an imagesignal of the red component (R) of the image data 30. The colorconversion processing portion 23 generates a black (K) image signal 31K,a cyan (C) image signal 31C, a magenta (M) image signal 31M, and ayellow (Y) image signal 31Y. The image signals 31K, 31C, 31M, and 31Yare input to light scanning apparatus (laser scanner units) 707K, 707C,707M, and 707Y, respectively.

((Image Forming Unit))

The image forming unit 701 includes four image forming portions 70 (70Y,70M, 70C, and 70K). The image forming portion 70Y forms a yellow imagewith the use of a yellow toner. The image forming portion 70M forms amagenta image with the use of a magenta toner. The image forming portion70C forms a cyan image with the use of a cyan toner. The image formingportion 70K forms a black image with the use of a black toner. Thesuffixes Y, M, C, and K in the reference symbols represent yellow,magenta, cyan, and black, respectively. In the following description,the suffixes Y, M, C, and K in the reference symbols may be omitted whenthe suffixes are not particularly necessary. The four image formingportions 70 have the same structure except for the color of the toner.

Each image forming portion 70 has a photosensitive drum (image bearingmember) 708 serving as a photosensitive member. The photosensitive drum708 rotates about a rotation axis 7 in a direction indicated by an arrowR1 of FIG. 1 when an image is formed. A charging device 709, the lightscanning apparatus 707, a developing device 710, a primary transferdevice 712, and a drum cleaning device (not shown) are arranged aroundthe photosensitive drum 708. An endless belt (hereinafter referred to as“intermediate transfer belt”) 711 serving as an intermediate transfermember is arranged below the photosensitive drum 708. The intermediatetransfer belt 711 is stretched over a drive roller 713, a tension roller715, and a secondary transfer opposite roller 714. The intermediatetransfer belt 711 rotates in a direction indicated by an arrow R2 ofFIG. 1 when an image is formed. On an upper surface of a horizontalportion of the intermediate transfer belt 711, a photosensitive drum708Y, a photosensitive drum 708M, a photosensitive drum 708C, and aphotosensitive drum 708K are arranged in the stated order along arotation direction R2 of the intermediate transfer belt 711. The primarytransfer device 712 is opposed to the photosensitive drum 708 throughthe intermediate transfer belt 711. A secondary transfer roller 716 isopposed to the secondary transfer opposite roller 714 through theintermediate transfer belt 711.

A section that is downstream of the image forming portion 70 and that isnear the intermediate transfer belt 711 is provided with patterndetectors (color misregistration measurement sensors) 726, which areoptical sensors configured to detect color misregistration measurementpatterns (toner images) that are formed on the intermediate transferbelt 711. The color misregistration measurement patterns are formed onthe intermediate transfer belt 711 before an image is formed on thesheet or between formation of an image and formation of another image,and are detected by the pattern detectors 726. Detection results of thepattern detectors 726 are used to correct a position and a magnificationof an image. A section that is upstream of the image forming portion 70and that is near the intermediate transfer belt 711 is provided with abelt cleaning device 717 configured to remove residual toner thatremains on the intermediate transfer belt 711 after secondary transfer.

A feeding cassette 718 configured to contain sheets S is arranged in alower part of the image forming apparatus 100. The sheets S are fed fromthe feeding cassette 718 by a pickup roller 719. The top sheet of thesheets fed by the pickup roller 719 is separated by a separation rollerpair 722, which is formed of a feed roller 722 a serving as a conveyingunit and a retard roller 722 b serving as a separation unit. The sheetfed by the separation roller pair 722 is conveyed to a registrationroller pair 723 by a plurality of conveying roller pairs 721 and 720. Aleading edge of the sheet is abutted against a nip of the registrationroller pair 723 that has stopped rotating. When the sheet is formed intoa loop, conveyance of the sheet is stopped once. The formation of theloop corrects skew feeding of the sheet. When the registration rollerpair 723 starts rotating, the sheet is conveyed to the secondarytransfer roller 716. A fixing device 724 is arranged downstream of thesecondary transfer roller 716 in a conveying direction of the sheet. Adischarge tray 725, on which the sheets with images formed thereon arestacked, is arranged downstream of the fixing device 724 in theconveying direction of the sheet.

(Image Forming Process)

An image forming process of the image forming apparatus 100 isdescribed. The photosensitive drums 708Y, 708M, 708C, and 708K rotate inthe direction indicated by the arrow R1 about rotation axes 7Y, 7M, 7C,and 7K thereof, respectively. In the yellow image forming portion 70Y, acharging device 709Y charges a surface of the photosensitive drum 708Yuniformly. The light scanning apparatus 707Y emits laser light(hereinafter referred to as “light beam”) that is modulated based on theyellow component image signal 31Y to form an electrostatic latent imageon the uniformly charged surface of the photosensitive drum 708Y. Adeveloping device 710Y develops the electrostatic latent image with theuse of a yellow toner to obtain a yellow toner image.

Next, in the magenta image forming portion 70M, a charging device 709Mcharges a surface of the photosensitive drum 708M uniformly. The magentalight scanning apparatus 707M starts emitting a light beam that ismodulated based on the magenta component image signal 31M when a firstpredetermined time period passes after timing at which the yellow lightscanning apparatus 707Y starts writing the electrostatic latent image ina sub scanning direction. The light scanning apparatus 707M scans thelight beam on the uniformly charged surface of the photosensitive drum708M in a main scanning direction to form an electrostatic latent image.A developing device 710M develops the electrostatic latent image withthe use of a magenta toner to obtain a magenta toner image.

Next, in the cyan image forming portion 70C, a charging device 709Ccharges a surface of the photosensitive drum 708C uniformly. The cyanlight scanning apparatus 707C starts emitting a light beam that ismodulated based on the cyan component image signal 31C when a secondpredetermined time period passes after timing at which the magenta lightscanning apparatus 707M starts writing the electrostatic latent image inthe sub scanning direction. The light scanning apparatus 707C scans thelight beam on the uniformly charged surface of the photosensitive drum708C in the main scanning direction to form an electrostatic latentimage. A developing device 710C develops the electrostatic latent imagewith the use of a cyan toner to obtain a cyan toner image.

Next, in the black image forming portion 70K, a charging device 709Kcharges a surface of the photosensitive drum 708K uniformly. The blacklight scanning apparatus 707K starts emitting a light beam that ismodulated based on the black component image signal 31K when a thirdpredetermined time period passes after timing at which the cyan lightscanning apparatus 707C starts writing the electrostatic latent image inthe sub scanning direction. The light scanning apparatus 707K scans thelight beam on the uniformly charged surface of the photosensitive drum708K in the main scanning direction to form an electrostatic latentimage. A developing device 710K develops the electrostatic latent imagewith the use of a black toner to obtain a black toner image. In thefirst embodiment, the first predetermined time period, the secondpredetermined time period, and the third predetermined time period arethe same, but may be set to different time periods depending on thestructure and conditions of the image forming apparatus 100.

The intermediate transfer belt 711 rotates in the direction indicated bythe arrow R2. The yellow toner image on the photosensitive drum 708Y istransferred onto the intermediate transfer belt 711 in a primarytransfer portion between the photosensitive drum 708Y and a primarytransfer device 712Y. Next, the magenta toner image on thephotosensitive drum 708M is transferred in an overlapping manner ontothe yellow toner image on the intermediate transfer belt 711 in aprimary transfer portion between the photosensitive drum 708M and aprimary transfer device 712M. Next, the cyan toner image on thephotosensitive drum 708C is transferred in an overlapping manner ontothe magenta toner image on the intermediate transfer belt 711 in aprimary transfer portion between the photosensitive drum 708C and aprimary transfer device 712C. Finally, the black toner image on thephotosensitive drum 708K is transferred in an overlapping manner ontothe cyan toner image on the intermediate transfer belt 711 in a primarytransfer portion between the photosensitive drum 708K and a primarytransfer device 712K. In this manner, the toner images of the fourcolors: yellow, magenta, cyan, and black are transferred onto theintermediate transfer belt 711 while being overlapped with one anotherin the stated order.

The sheet fed from the feeding cassette 718 waits at the registrationroller pair 723. The registration roller pair 723 starts rotating at theright timing such that the toner images on the intermediate transferbelt 711 match a position of the sheet. The sheet is conveyed to asecondary transfer portion between the secondary transfer roller 716 andthe secondary transfer opposite roller 714 by the registration rollerpair 723. The four-color toner images on the intermediate transfer belt711 are transferred at once onto the sheet by the secondary transferroller 716. The sheet with the toner images transferred thereon isconveyed to the fixing device 724 serving as an image fixing unit. Thefixing device 724 heats and pressurizes the sheet to fix the tonerimages to the sheet and form a full-color image on the sheet. The sheetwith the image formed thereon is discharged onto the discharge tray 725.

(Light Scanning Apparatus)

The light scanning apparatus 707 serving as light beam emitting devicesis described next. FIG. 2 is a plan view for schematically illustratingcomponents that are arranged inside the light scanning apparatus 707.The four light scanning apparatus 707Y, 707M, 707C, and 707K have thesame structure. The light scanning apparatus 707 includes asemiconductor laser chip (hereinafter referred to as “light source”) 10,a collimator lens 11, a rotary polygon mirror 12, a motor 17 configuredto rotate the rotary polygon mirror 12, an fθ lens 15, and a reflectingmirror 16. The light scanning apparatus 707 also includes a beamdetector (hereinafter referred to as “BD”) 14, and a photodiode(hereinafter referred to as “PD”) 13 serving as a light receiver. Thelight scanning apparatus 707 further includes a light source driveportion (laser driver) 18 configured to drive the light source 10, and amotor drive portion 19 configured to drive the motor 17.

The light source 10 is an edge emitting laser configured to emit lightbeams in two directions from half mirrors formed on both end surfaces,respectively. The light source 10 is configured to emit a light beam asfront light toward the collimator lens 11, and also emit a light beam asrear light toward the PD 13. The front light is guided to the surface ofthe photosensitive drum 708 to form an electrostatic latent image on thesurface of the photosensitive drum 708. The rear light is emitted with alight intensity that is a predetermined ratio of a light intensity ofthe front light to enter the PD 13 serving as a detector configured todetect the light intensity. In automatic power control (hereinafterreferred to as “APC”) of the light source 10, when receiving the rearlight, the PD 13 serving as a photoelectric conversion unit converts therear light into an electrical signal. The PD 13 outputs the electricalsignal as a detection signal (hereinafter referred to as “PD signal”) 32to the light source drive portion 18. The light source drive portion 18adjusts the light intensity of the light beam emitted from the lightsource 10 based on the PD signal 32. The light source 10 in the firstembodiment is not limited to an edge emitting laser, but may be asurface emitting laser, for example, a vertical cavity surface emittinglaser (VCSEL) or a vertical external cavity surface emitting laser(VECSEL). Moreover, the light source 10 may be a single beam generatingunit configured to emit a single light beam, or a multi-beam generatingunit configured to emit a plurality of light beams.

The light source 10 emits the light beam based on the image signal 31 ofthe corresponding color component. The collimator lens 11 converts thelight beam emitted from the light source 10 into a substantiallyparallel light beam. The motor drive portion 19 outputs anacceleration/deceleration signal (hereinafter referred to as “drivesignal”) 34 to the motor 17, which is integrally formed with the rotarypolygon mirror 12 serving as a deflector, to rotate the rotary polygonmirror 12 at a predetermined speed in a direction indicated by an arrowR3 of FIG. 2. The light beam forms an image on a reflecting surface ofthe revolving rotary polygon mirror 12, and is deflected by the rotarypolygon mirror 12. The light beam reflected by the rotary polygon mirror12 passes through the fθ lens 15, is reflected by the reflecting mirror16, and forms an image as a light spot that travels on thephotosensitive drum 708 in the main scanning direction indicated by anarrow MS of FIG. 2 at a constant speed. The fθ lens 15 converts thelight beam that has been scanned at a constant angular velocity by therotary polygon mirror 12 into the light spot that travels in the mainscanning direction MS at the constant speed.

The light beam emitted from the light source 10 outside an image formingarea is reflected by the rotary polygon mirror 12, and enters the BD 14.The BD 14 serving as a beam detector receives the light beam, andoutputs a synchronization signal (hereinafter referred to as “BDsignal”) 33 for making constant a writing start position of anelectrostatic latent image on the photosensitive drum 708, which isscanned with the light beam at a constant position in the main scanningdirection. The BD signal 33 is input to the motor drive portion 19 andthe image controller 20. The motor drive portion 19 executes feedbackcontrol of a rotation speed of the motor 17 such that a period of the BDsignal 33 is stabilized at a predetermined period. The image controller20 outputs the image signal 31 to the light source drive portion 18based on the BD signal 33. After executing the APC, the light sourcedrive portion 18 determines light beam emission start timing based onthe BD signal 33, and starts writing an image. In this manner, writingstart positions in the main scanning direction of the image are matched.

The light source drive portion 18 outputs, to the light source 10, adrive signal 35 for flashing the light source 10 based on the imagesignal 31 output from the image controller 20. The light source driveportion 18 drives the light source 10 based on the image signal 31 attiming at which the light beam is scanned on an image forming area ofthe photosensitive drum 708. The light source 10 emits the light beammodulated based on the image signal 31. The light spot of the light beamemitted from the light source 10 and deflected by the rotary polygonmirror 12 travels on the surface of the photosensitive drums 708, whichis charged uniformly by the charging device 709, in parallel to therotation axis 7 of the photosensitive drum 708 in a linear pattern at aconstant speed. An electric potential on the surface of thephotosensitive drum 708 varies depending on the intensity of the lightbeam. The photosensitive drum 708 is repeatedly scanned with the lightbeam in the main scanning direction MS while being rotated in thesub-scanning direction R1, which is perpendicular to the main scanningdirection MS, with the result that the electrostatic latent image isformed in the sub-scanning direction R1.

(Magnification Correction of Image)

However, depending on an individual difference of the fθ lens 15, atemperature change, or a change with time, the images of the respectivecolors may not be overlapped with one another correctly in the mainscanning direction MS to cause color misregistration in some cases. Inorder to avoid the color misregistration, a magnification in the mainscanning direction MS of an image is corrected. Now, referring to FIG.3, magnification correction on an image in the main scanning directionMS is described. FIG. 3 is a flowchart for illustrating magnificationcorrection operation on an image in the main scanning direction MS.

((Profile Magnification Correction))

First, in order to correct color misregistration caused by a componentthat cannot be completely corrected by the fθ lens 15 or an individualdifference at the time of production, the light beam of the lightscanning apparatus 707 is measured in advance by a profile measuringdevice 40, and a measurement result is stored in the memory 22. At thetime of image formation, the measurement result is read from the memory22, and the image data is corrected based on a magnification of aprofile determined based on the measurement result. The profile ismagnification property information indicating a magnification withrespect to a position (hereinafter referred to as “scanning position”) Xin the main scanning direction MS for each light scanning apparatus 707.A correction amount for correcting the magnification of the profile ishereinafter referred to as “profile magnification (first magnification)Mag_P”. Correction of the image data based on the profile magnificationMag_P is referred to as “profile magnification correction”. The profilemagnification correction allows the light spot that forms an image onthe photosensitive drum 708 to travel at the constant speed. Ameasurement operation using the profile measuring device 40 is performedbefore the light scanning apparatus 707 is assembled into the imageforming apparatus 100.

FIG. 4A and FIG. 4B are an explanatory view and an explanatory diagramof the profile measuring device 40, respectively. FIG. 4A is aperspective view of the profile measuring device 40. The profilemeasuring device 40 includes a front detector 41, a center detector 42,and a rear detector 43, which are arrayed in line in the main scanningdirection MS. FIG. 4B is a diagram for illustrating a positionalrelationship among the BD 14, which is provided in the light scanningapparatus 707, and the front detector 41, the center detector 42, andthe rear detector 43, which are provided in the profile measuring device40. From upstream to downstream in the main scanning direction MS, theBD 14, the front detector 41, the center detector 42, and the reardetector 43 are arranged in the stated order at the same interval Sx. Inthe first embodiment, the interval (inter-detector distance) Sx is setto 100 mm, but the present invention is not limited thereto.

When the light beam emitted from the light scanning apparatus 707 passesabove the front detector 41, the center detector 42, and the reardetector 43, each of the front detector 41, the center detector 42, andthe rear detector 43 outputs an electrical signal. Times Tf, Tc, and Trat which the light beam passes above the front detector 41, the centerdetector 42, and the rear detector 43, respectively, are measured withreference to the BD signal 33 output from the BD 14. The time Tf is thetime it takes for the light beam to travel from the BD 14 to the frontdetector 41. The time Tc is the time it takes for the light beam totravel from the BD 14 to the center detector 42. The time Tr is the timeit takes for the light beam to travel from the BD 14 to the reardetector 43. Displacement amounts ΔSf, ΔSc, and ΔSr of the light beamwith respect to the scanning position X in the main scanning directionMS are determined based on the times Tf, Tc, and Tr. The scanningposition X has its origin (X=0) at the position of the center detector42. The scanning position X of the front detector 41 is −Sx (X=−Sx). Thescanning position X of the rear detector 43 is Sx (X=Sx). Thedisplacement amount ΔSf indicates a displacement amount from an idealscanning position at a time when the light beam is at the scanningposition X=−Sx. The displacement amount ΔSc indicates a displacementamount from an ideal scanning position at a time when the light beam isat the scanning position X=0. The displacement amount ΔSr indicates adisplacement amount from an ideal scanning position at a time when thelight beam is at the scanning position X=Sx. When a light beam scanningspeed is represented by V, the displacement amounts ΔSf, ΔSc, and ΔSrare expressed by the following expressions.ΔSf=Sx−(Tf×V)ΔSc=(2×Sx)−(Tc×V)ΔSr=(3×Sx)−(Tr×V)

For example, when the scanning speed V is 1 mm/μs, the interval(inter-detector distance) Sx is 100 mm, and the time Tf at which thelight beam passes above the front detector 41 is 101 μs, thedisplacement amount ΔSf is −1 mm.

The measured values of the displacement amounts ΔSf, ΔSc, and ΔSr arestored in the memory (storage portion) 22 of the image controller 20.The values of the displacement amounts ΔSf, ΔSc, and ΔSr are used incorrecting the magnification of a pixel in the main scanning directionMS based on the profile during the image formation (hereinafter referredto as “profile magnification correction”). When image forming operationis started, the CPU 21 starts the magnification correction operation inaccordance with the flowchart of FIG. 3. The CPU 21 reads the values ofthe displacement amounts ΔSf, ΔSc, and ΔSr from the memory 22. The CPU21 calculates a displacement amount Y with respect to the scanningposition X based on the values of the displacement amounts ΔSf, ΔSc, andΔSr. FIG. 5 is a graph for showing the displacement amount Y withrespect to the scanning position X. When measurement results of thethree detectors: the front detector 41, the center detector 42, and therear detector 43 are used, an approximation of the displacement amount Ywith respect to the scanning position X is expressed as a quadratic. Thedisplacement amount Y with respect to the scanning position X isexpressed by Expression 1 below.

$\begin{matrix}{Y = {{\frac{{\Delta\;{Sr}} + {\Delta\;{Sf}} - {2\Delta\;{Sc}}}{2 \times S_{x}^{2}}x^{2}} + {\frac{{\Delta\;{Sr}} - {\Delta\;{Sf}}}{2 \times S_{x}}x} + {\Delta\;{Sc}}}} & ( {{Expression}\mspace{14mu} 1} )\end{matrix}$

In the first embodiment, a scanning area in the main scanning directionMS is divided into eight areas (hereinafter referred to as “blocks”)(Block 1 to Block 8). A displacement amount Mag_Block(N) is set for eachblock. In the first embodiment, Block 1 to Block 8 are set to have thesame width, and Block 1 to Block 8 are arranged at equal intervals.However, the width (interval) of the block may be set smaller towardends with a larger variation in displacement amount. The displacementamount Mag_Block(N) of each block is expressed by Expression 2 below.

$\begin{matrix}{{{Mag\_ Block}(N)} = \frac{{Y(n)} - {Y( {n + 1} )}}{{Block\_ Width}(N)}} & ( {{Expression}\mspace{14mu} 2} )\end{matrix}$

In Expression 2, N represents a block number, and n represents acoordinate at the left end of Block N. Y(n) represents a displacementamount at the left end of Block N, and Y(n+1) represents a displacementamount at the left end of Block N+1. Block_Width(N) represents a widthof Block N.

For example, when the measurement results of the displacement amountsare ΔSf=−0.32 mm, ΔSc=0 mm, and ΔSr=−0.32 mm, and a scanning area of 320mm is divided into eight blocks, the profile magnification Mag_P asshown in FIG. 6 is obtained. FIG. 6 is a graph for showing the profilemagnification Mag_P (%) with respect to the scanning position X mm. TheCPU 21 calculates the profile magnification Mag_P based on the values ofthe displacement amounts ΔSf, ΔSc, and ΔSr stored in the memory 22 (StepS1). In the first embodiment, the profile magnification Mag_P is set foreach block based on the displacement amount Mag_Block(N) of each block.The CPU 21 starts the profile magnification correction for correctingthe image data based on the profile magnification set for each block(Step S2). Through the correction of the image data at each scanningposition X based on the profile magnification Mag_P set for each block,the color misregistration caused by the component that cannot becompletely corrected by the fθ lens 15 or an individual difference atthe time of production can be corrected. When a calculation speed of theCPU 21 with respect to a printing speed is sufficiently fast, theprofile magnification Mag_P may be set for each pixel based onExpression 1. The CPU 21 executes the calculation of the profilemagnification Mag_P and the profile magnification correction, which havebeen described above, for the light scanning apparatus 707 of eachcolor.

((Color Misregistration Magnification Correction))

In order to correct a magnification that varies depending on thetemperature, the change with time, and other such factors, the CPU 21corrects the image data. A correction amount for correcting themagnification that varies depending on the temperature, the change withtime, and other such factors are hereinafter referred to as “colormisregistration magnification (second magnification) Mag_I”. Thecorrection of the image data based on the color misregistrationmagnification Mag_I is referred to as “color misregistrationmagnification correction”. In the color misregistration magnificationcorrection, the color misregistration measurement patterns are formed onthe intermediate transfer belt 711 with the use of the image data thathas been subjected to the profile magnification correction. The CPU 21detects the color misregistration measurement patterns with the patterndetectors 726 (Step S3). FIG. 7 is a diagram for illustrating colormisregistration measurement patterns 727 and 728 and pattern detectors726 a and 726 b. Color misregistration magnification correction of atarget color image corrects the displacement amount with respect to areference color image. Yellow is hereinafter referred to as “referencecolor”, and colors (magenta, cyan, and black) other than the referencecolor are hereinafter referred to as “target colors”. In steps similarto those in the case where the toner images are transferred to thesheet, the color misregistration measurement patterns as the tonerimages are formed on the intermediate transfer belt 711 by the primarytransfer device 712, and the color misregistration measurement patternsare detected by the pattern detectors 726 provided at an end of theintermediate transfer belt 711. As illustrated in FIG. 7, the patterndetectors 726 include a front pattern detector 726 a on an upstream side(hereinafter referred to as “front side”) in the main scanning directionMS, and a rear pattern detector 726 b on a downstream side (hereinafterreferred to as “rear side”) in the main scanning direction MS. At eachof positions on the intermediate transfer belt 711 that pass below thefront pattern detector 726 a and the rear pattern detector 726 b, thecolor misregistration measurement patterns 727 and 728, which are thesame patterns, are formed. The color misregistration measurementpatterns 727 on the front side include reference color patterns (firstpatterns) 727 a and 727 c having oblique line shapes, and target colorpatterns (second patterns) 727 b and 727 d having oblique line shapes.Similarly, the color misregistration measurement patterns 728 on therear side include reference color patterns (first patterns) 728 a and728 c having oblique line shapes, and target color patterns (secondpatterns) 728 b and 728 d having oblique line shapes. An intervalbetween the reference color patterns 727 a and 727 c (728 a and 728 c),and an interval between the target color patterns 727 b and 727 d (728 band 728 d) are measured to measure the displacement amount in the mainscanning direction MS of an image of a target color with respect to animage of the reference color.

Now, there is described measurement of the displacement amount(positional displacement amount) in the main scanning direction MS of animage of magenta as a target color with respect to an image of yellow asthe reference color. The front pattern detector 726 a detects aninterval Ref(F) between yellow reference color patterns 727 a and 727 cand an interval Tar(F) between magenta target color patterns 727 b and727 d. Similarly, the rear pattern detector 726 b detects an intervalRef(R) between yellow reference color patterns 728 a and 728 c and aninterval Tar(R) between magenta (M) target color patterns 728 b and 728c.

The displacement amount ΔRf of magenta with respect to yellow on thefront side is expressed as follows.ΔRf=(Tar(F)−Ref(F))/2

The displacement amount ΔRr of magenta with respect to yellow on therear side is expressed as follows.ΔRr=(Tar(R)−Ref(R))/2

The scanning position X at which the front pattern detector 726 a isarranged is represented by −Rx, and the scanning position X at which therear pattern detector 726 b is arranged is represented by +Rx. FIG. 8 isa graph for showing the displacement amount with respect to the scanningposition X. The number of points of measurement of the displacementamount is two, and hence the displacement amount Y in the main scanningdirection MS of the target color with respect to the reference color isexpressed as the following approximation of a linear function as shownin FIG. 8.

$Y = {{\frac{{\Delta\;{Rr}} - {\Delta\;{Rf}}}{2 \times R_{x}}x} + \frac{{\Delta\;{Rr}} + {\Delta\;{Rf}}}{2}}$

In accordance with the approximation of the linear function above, arate of change of the displacement amount Y is constant in all areas ofthe scanning position X in the main scanning direction MS. A Δdisplacement amount, which is a difference between the displacementamount ΔRf on the front side and the displacement amount ΔRr on the rearside in the main scanning direction MS, is obtained from an interval(2×Rx) between the front pattern detector 726 a and the rear patterndetector 726 b by the following expression.

${\Delta\;{Displacement}\mspace{14mu}{Amount}} = {{{\frac{{\Delta\;{Rr}} - {\Delta\;{Rf}}}{2 \times R_{x}}( {- R_{x}} )} + \frac{{\Delta\;{Rr}} - {\Delta\;{Rf}}}{2} - ( {{\frac{{\Delta\;{Rr}} - {\Delta\;{Rf}}}{2 \times R_{x}}( {+ R_{x}} )} + \frac{{\Delta\;{Rr}} + {\Delta\;{Rf}}}{2}} )} = {{\Delta\;{Rf}} - {\Delta\;{Rr}}}}$

The CPU 21 calculates the color misregistration magnification Mag_Ibased on the displacement amount ΔRf and the displacement amount ΔRr(Step S4). The color misregistration magnification Mag_I (%) isexpressed as Expression 3 below.

$\begin{matrix}{{Mag\_ I} = {\frac{{\Delta\;{Rf}} - {\Delta\;{Rr}}}{2 \times R_{x}} \times 100}} & ( {{Expression}\mspace{14mu} 3} )\end{matrix}$

For example, it is assumed that Rx=100 mm, that the interval between thefront pattern detector 726 a and the rear pattern detector 726 b is 200mm (2×Rx=200 mm), that ΔRf=2 mm, and that ΔRr=−2 mm. Based on Expression3, the color misregistration magnification Mag_I is 2%. The detectionand the calculation of the color misregistration measurement patterns727 and 728 are performed also for remaining cyan and black to determinethe color misregistration magnification Mag_I for each color. Colormisregistration magnification correction for yellow, which is thereference color, is not performed, and color misregistrationmagnification correction is performed so as to match colors other thanyellow to the reference color. In this manner, the CPU 21 serving as acolor misregistration magnification generating portion generates thecolor misregistration magnification (second magnification) Mag_I of thetarget color image with respect to the reference color image based ondetection results of the color misregistration measurement patterns 727and 728.

((Composite Magnification Correction))

The CPU 21 composites the profile magnification Mag_P and the colormisregistration magnification Mag_I, which have been determined asdescribed above, to calculate a composite magnification (thirdmagnification) Mag with respect to each scanning position X (Step S5).The composite magnification Mag is expressed as Expression 4 below.

$\begin{matrix}{{Mag} = {( {{( {1 + \frac{Mag\_ P}{100}} )( {1 + \frac{Mag\_ I}{100}} )} - 1} ) \times 100}} & ( {{Expression}\mspace{14mu} 4} )\end{matrix}$

The image data is corrected based on the composite magnification Mag,which is determined as described above (Step S6). The image is formed onthe sheet based on the corrected image data (Step S7). An image withreduced color misregistration can be formed by correcting the image databased on the composite magnification Mag.

FIG. 9 is a graph for showing the profile magnification Mag_P and thecomposite magnification Mag with respect to the scanning position X. Thecomposite magnification Mag is generated by compositing the colormisregistration magnification Mag_I of +2% with the profilemagnification Mag_P. Along with enlargement in the main scanningdirection MS of the image, which is caused by the correction of theimage data based on the color misregistration magnification Mag_I, acorrected pixel position (first pixel position) based on the compositemagnification Mag is displaced in the main scanning direction MS withrespect to a corrected position of the image data based on the profilemagnification Mag_P. FIG. is an enlarged graph of an end of the profilemagnification Mag_P and the composite magnification Mag with respect tothe scanning position X. As shown in FIG. 10, the displacement in themain scanning direction MS of the corrected pixel position (first pixelposition) of the composite magnification Mag with respect to a correctedpixel position (second pixel position) of the profile magnificationMag_P is small, and hence does not pose a problem.

(Magnification Correction in Light Scanning Apparatus without fθ Lens)

In recent years, however, in order to reduce cost, the fθ lens 15 isomitted from the light scanning apparatus 707. The fθ lens 15 has afunction of converting the light beam that rotates at the constantangular velocity into the light spot that travels on the photosensitivedrum 708 at the constant speed. In the light scanning apparatus 707without the fθ lens 15, image displacement is corrected through theprofile magnification correction, and hence the profile magnification(correction amount) becomes larger as compared to the case of the lightscanning apparatus 707 with the fθ lens 15. When the profilemagnification (correction amount) is increased, displacement of a pixelposition in the profile magnification correction poses a problem. FIG.11 is a graph for showing a profile magnification Mag_P and a compositemagnification Mag with respect to the scanning position X in a case ofno fθ lens 15. FIG. 12 is an enlarged graph of an end of the profilemagnification Mag_P and the composite magnification Mag with respect tothe scanning position X in the case of no fθ lens 15. As shown in FIG.11, the profile magnification Mag_P of the image forming apparatus 100without the fθ lens 15 is significantly increased as compared to theprofile magnification Mag_P of the image forming apparatus 100 with thefθ lens 15, which is shown in FIG. 9. Displacement in the main scanningdirection MS of the corrected pixel position (first pixel position) ofthe composite magnification Mag with respect to the corrected pixelposition (second pixel position) of the profile magnification Mag_P,which is shown in FIG. 12, is large as compared to the displacement inthe case of the image forming apparatus 100 with the fθ lens 15, whichis shown in FIG. 10. Due to the displacement of the corrected pixelposition, the color misregistration cannot be corrected sufficiently insome cases.

To address this problem, in the first embodiment, before the profilemagnification Mag_P and the color misregistration magnification Mag_Iare composited to generate the composite magnification Mag, the profilemagnification Mag_P is corrected based on the color misregistrationmagnification Mag_I. As a result, even when the profile magnificationMag_P is large, the color misregistration of an image can be reduced.

((Correction of Profile Magnification Based on Color MisregistrationMagnification))

FIG. 13 is a flowchart for illustrating magnification correctionoperation on an image in the main scanning direction MS in the case ofno fθ lens 15. Steps S1 to S4 of FIG. 13 are the same as Steps S1 to S4of FIG. 3, and hence descriptions thereof are omitted. Based on thecolor misregistration magnification Mag_I determined based on Expression3, the displacement amount Y caused by the profile magnificationdetermined based on Expression 1 is corrected (Step S11). The CPU 21calculates the displacement amount Y with respect to the scanningposition X based on the values of the displacement amounts ΔSf, ΔSc, andΔSr and the color misregistration magnification Mag_I. The displacementamount Y with respect to the scanning position X is expressed byExpression 5 below.

$\begin{matrix}{Y = {{\frac{{\Delta\;{Sr}} + {\Delta\;{Sf}} - {2\Delta\;{Sc}}}{2 \times S_{x}^{2}}( {x \times ( {1 + \frac{Mag\_ I}{100}} )} )^{2}} + {\frac{{\Delta\;{Sr}} - {\Delta\;{Sf}}}{2 \times S_{x}}( {x + ( {1 + \frac{Mag\_ I}{100}} )} )} + {\Delta\;{Sc}}}} & ( {{Expression}\mspace{14mu} 5} )\end{matrix}$

FIG. 14 is a graph for showing the corrected displacement amount Y withrespect to the scanning position X. As shown in FIG. 14, the scanningposition X is corrected based on the color misregistration magnificationMag_I. The displacement amount Mag_Block(N) of each block is determinedby substituting the displacement amount Y determined based on Expression5 into Expression 2. The CPU 21 sets the profile magnification Mag_P foreach block based on the displacement amount Mag_Block(N) of each block.In this manner, the CPU 21 corrects the profile magnification Mag_Pbased on the color misregistration magnification (Step S12).

In the first embodiment, Expression 1 is modified with the colormisregistration magnification Mag_I to obtain Expression 5. However,Expression 2 may be modified with the color misregistrationmagnification Mag_I to obtain the following expression for determiningthe displacement amount Mag_Block(N) of each block.

${{Mag\_ Block}(N)} = \frac{{Y(n)} - {Y( {n + 1} )}}{{Block\_ Width}{(N) \div ( {1 + \frac{Mag\_ I}{100}} )}}$

The CPU 21 may set the profile magnification Mag_P for each block basedon the displacement amount Mag_Block(N) of each block that is correctedbased on the color misregistration magnification Mag_I. Also in thismanner, the CPU 21 can correct the profile magnification Mag_P based onthe color misregistration magnification.

The CPU 21 calculates the composite magnification Mag of the profilemagnification Mag_P corrected based on the color misregistrationmagnification Mag_I and the color misregistration magnification Mag_Ibased on Expression 4 (Step S13). FIG. 15 is a graph for showing theprofile magnification Mag_P and the corrected composite magnificationMag with respect to the scanning position X. FIG. 16 is an enlargedgraph of an end of the profile magnification Mag_P and the correctedcomposite magnification Mag with respect to the scanning position X. Thecorrected composite magnification Mag shown in FIG. 15 and FIG. 16 isgenerated by compositing the profile magnification Mag_P corrected basedon the color misregistration magnification Mag_I and the colormisregistration magnification Mag_I. The corrected compositemagnification Mag has a smaller error from an ideal magnification ascompared to the composite magnification Mag shown in FIG. 12, and hencethe color misregistration of the color image formed on the sheet can bereduced.

FIG. 17A and FIG. 17B are diagrams for illustrating relationshipsbetween an output image and the corrected pixel position. FIG. 17A is adiagram for illustrating a relationship between the output image thathas been subjected to the profile magnification correction without thecolor misregistration magnification correction and the corrected pixelposition. FIG. 17B is a diagram for illustrating a relationship betweenthe output image that has been subjected to composite magnificationcorrection of the profile magnification correction and the colormisregistration magnification correction and the corrected pixelposition. When the composite magnification correction is executed, asillustrated in FIG. 17B, the output image and the corrected pixelposition in the profile magnification correction are both scaled withthe color misregistration magnification in the main scanning directionMS. FIG. 17C is a diagram for illustrating a relationship between theoutput image that has been subjected to the composite magnificationcorrection of the profile magnification correction corrected by thecolor misregistration magnification correction and the colormisregistration magnification correction and the corrected pixelposition. When the composite magnification correction corrected by thecolor misregistration magnification correction is executed, asillustrated in FIG. 17C, the output image is scaled, but the correctedpixel position in the profile magnification correction is unchanged.Therefore, through the execution of the corrected compositemagnification correction, the image can be corrected at an appropriatemagnification, and the color misregistration can be reduced.

The CPU 21 corrects the image data based on the corrected compositemagnification Mag (Step S14). The image is formed on the sheet based onthe corrected image data (Step S15). Through the correction of the imagedata based on the composite magnification Mag of the profilemagnification correction corrected by the color misregistrationmagnification correction and the color misregistration magnificationcorrection, the image with the reduced color misregistration can beformed.

According to the first embodiment, there can be reduced the colormisregistration in the case where the profile magnification (firstmagnification) and the color misregistration magnification (secondmagnification) of the image in the main scanning direction are correctedin combination.

Second Embodiment

Now, a second embodiment of the present invention is described. In thesecond embodiment, structures similar to those in the first embodimentare denoted by similar reference symbols, and descriptions thereof areomitted. The image forming apparatus 100, the image forming process, andthe light scanning apparatus 707 in the second embodiment are similar tothose in the first embodiment, and hence descriptions thereof areomitted. In the second embodiment, the composite magnificationcorrection by the image controller 20 is described. The calculation ofthe profile magnification, the color misregistration magnification, andthe composite magnification is similar to that in the first embodiment,and hence a description thereof is omitted.

FIG. 18 is a flowchart for illustrating image forming control operation.FIG. 19 is a block diagram of the image controller 20. The imagecontroller 20 includes a CPU 21, a memory 22, an image data holdingportion 24, a profile magnification holding portion 25, a colormisregistration magnification holding portion 26, a pixel sizecalculating portion 27, a magnification compositing portion 28, and animage signal output portion 29. The CPU executes image forming operationas programmed by a program that is stored in the memory 22. When theimage forming control operation is started, the CPU 21 stores, in theimage data holding portion 24, the image data 30 output from the imagereading unit 700. The CPU 21 stores, in the profile magnificationholding portion 25, the profile magnification with respect to thescanning position X, which has been obtained based on the measurementresult of the profile measuring device 40. The CPU 21 stores, in thecolor misregistration magnification holding portion 26, the colormisregistration magnification obtained from the detection result of thepattern detectors 726.

The CPU 21 determines whether a TOP signal 36 has been input to thepixel size calculating portion 27 (Step S21). The TOP signal 36 is asynchronization signal of the light beam in the sub scanning directionR1. The TOP signal is used to print the top (first line) of an image atan appropriate position of the sheet. When the TOP signal has been input(YES in Step S21), the CPU 21 determines whether a BD signal 33 has beeninput to the pixel size calculating portion 27 (Step S22). When thelight beam deflected by the rotary polygon mirror 12 enters the BD 14,the BD 14 outputs the BD signal 33 to the pixel size calculating portion27. When the BD signal 33 has been input (YES in Step S22), the pixelsize calculating portion 27 starts processing on the first pixel of thefirst line with reference to the BD signal 33. First, the pixel sizecalculating portion 27 reads the profile magnification of the firstpixel from the profile magnification holding portion 25 (Step S23).Next, the pixel size calculating portion 27 reads the colormisregistration magnification from the color misregistrationmagnification holding portion 26 (Step S24). The pixel size calculatingportion uses the magnification compositing portion 28 to calculate thecomposite magnification of the profile magnification and the colormisregistration magnification with the use of Expression 4 (Step S25).

The pixel size calculating portion 27 calculates a pixel size based onthe image data, the composite magnification, and an error amount at thetime when the previous one pixel size is determined (Step S26). In otherwords, the pixel size calculating portion 27 determines to what size onepixel is enlarged, reduced, or maintained in terms of the magnification.The pixel size calculating portion 27 outputs the calculated pixel sizeto the image signal output portion 29. The image signal output portion29 outputs, to the light source drive portion 18, the image signal 31corresponding to one pixel based on the calculated pixel size and theimage data in the image data holding portion 24 (Step S27). The lightsource drive portion 18 generates the drive signal 35 based on the imagesignal 31, and outputs the drive signal 35 to the light source 10. Thelight source 10 emits the light beam based on the drive signal 35 toform a latent image corresponding to one pixel on the photosensitivedrum 708. The CPU 21 determines whether output of image signals 31corresponding to one line has been completed (Step S28). When the outputof the image signals 31 corresponding to one line has not been completed(NO in Step S28), the processing returns to Step S23. The CPU 21similarly corrects a pixel size of the next pixel. When the output ofthe image signals 31 corresponding to one line has been completed (YESin Step S28), the CPU 21 determines whether the image formation has beencompleted (Step S29). When the image formation has not been completed(NO in Step S29), the processing returns to Step S22, and image signals31 for the next line are generated. When the image formation has beencompleted (YES in Step S29), the CPU 21 ends the image formation.

The determination of the size of one pixel and the output of the imagesignal 31 are described with reference to FIG. 20A, FIG. 20B, and FIG.20C. FIG. 20A, FIG. 20B, and FIG. 20C are diagrams for illustrating theimage signal 31 and the pixel size. The image data 30 is generated as abit data group obtained by being divided by a predetermined integervalue for each pixel. One pixel is formed of a predetermined integernumber of bit data (tiny pixel pieces). Inserting (add) at least one bitdata into a bit data group of one pixel, and extracting (deleting) atleast one bit data from the bit data group of one pixel are referred toas “bit data insertion/extraction”. The pixel size is a size (length) ofone pixel in the main scanning direction MS. The size of one pixel inthe main scanning direction MS is changed by the bit datainsertion/extraction. When the size of one pixel is enlarged in the mainscanning direction MS, at least one bit data is inserted into the bitdata group of one pixel to increase the size of one pixel. When the sizeof one pixel is reduced in the main scanning direction MS, at least onebit data is extracted from the bit data group of one pixel to reduce thesize of one pixel. In this manner, the magnification of the image in themain scanning direction MS can be changed by the bit datainsertion/extraction.

For example, when the predetermined integer value is 100, one pixel ofthe image data 30 is formed of 100 bit data. When the magnificationcorrection is not required, one pixel is formed of 100 bit data asillustrated in FIG. 20A. When the image is enlarged by +30%magnification correction, 30 bit data is added to the original onepixel, with the result that the corrected one pixel is formed of 130 bitdata as illustrated in FIG. 20B. When the light beam is emitted from thelight source 10 based on the image signal 31 formed of 130 bit data perpixel, an enlarged latent image is formed on the photosensitive drum708. ON the contrary, when the image is reduced by −30% magnificationcorrection, 30 bit data is deleted from the original one pixel, with theresult that the corrected one pixel is formed of 70 bit data asillustrated in FIG. 20C. When the light beam is emitted from the lightsource 10 based on the image signal 31 formed of 70 bit data per pixel,a reduced latent image is formed on the photosensitive drum 708.

FIG. 21 is a diagram for illustrating magnifications and pixel sizes incontinuous output operation of image signals 31. A pixel size of onepixel without magnification correction is 100 bit data. The colormisregistration magnification is +2%. The profile magnification is −25%for the first Block 0, and −20% for the next Block 1. The pixel size ofthe first pixel is changed, based on the composite magnification of−23.5% that has been calculated by the magnification compositing portion28, from 100 bit data to 77 bit data by the pixel size calculatingportion 27. The pixel size is 76.5 bit data in calculation, but isdetermined as 77 bit data because the image signal 31 is output in unitsof 1 bit data. Therefore, an error of −0.5 bit data is generated. Theerror of −0.5 bit data is used when a pixel size of the next pixel isdetermined. The next one pixel is also 76.5 bit data in calculation, butis output as 76 bit data based on the error of −0.5 bit data. Suchcalculation of the pixel size is repeated such that the image as a wholeis scaled to a target magnification. Further, at around the center ofFIG. 21, the block of the profile magnification, which has beendescribed in the first embodiment, is switched from Block 0 to Block 1.When the block has been switched from Block 0 to Block 1, the profilemagnification of −20% of Block 1 is read from the profile magnificationholding portion 25. The magnification compositing portion 28 compositesthe profile magnification of −20% and the color misregistrationmagnification of +2% to generate the composite magnification of −18.4%.In Block 1, the pixel size is determined by the pixel size calculatingportion 27 based on the composite magnification of −18.4%. Throughinsertion/extraction of bit data for each pixel based on the pixel sizecalculated as described above, the image data can be corrected to reducethe color misregistration of the image.

However, as in the first embodiment, the composite magnificationcorrection is performed in synchronization with one pixel of the outputimage, and hence the corrected pixel position (first pixel position) ofthe composite magnification correction may be displaced from thecorrected pixel position (second pixel position) of the profilemagnification correction in some cases. When the profile magnification(correction amount) is large, the color misregistration may occur. Toaddress this problem, in the second embodiment, the image data iscorrected based on the displacement of the corrected pixel positionbetween the composite magnification correction and the profilemagnification correction. FIG. 22 is a flowchart for illustrating imageforming control operation based on the displacement of the correctedpixel position. FIG. 23 is a block diagram of the image controller 20based on the displacement of the corrected pixel position. The imagecontroller 20 includes a CPU 21, a memory 22, an image data holdingportion 24, a profile magnification holding portion 25, a colormisregistration magnification holding portion 26, a first pixel sizecalculating portion 51, a second pixel size calculating portion 52, amagnification compositing portion 28, and an image signal output portion29. The CPU 21 executes the image forming operation as programmed by aprogram that is stored in the memory 22. When the image forming controloperation is started, the CPU 21 stores the image data in the image dataholding portion 24. The CPU 21 stores, in the profile magnificationholding portion 25, the profile magnification with respect to thescanning position X, which is obtained from the measurement result ofthe profile measuring device 40. The CPU 21 stores, in the colormisregistration magnification holding portion 26, the colormisregistration magnification obtained from the detection result of thepattern detectors 726.

The CPU 21 determines whether a TOP signal 36 has been input to thefirst pixel size calculating portion 51 and the second pixel sizecalculating portion 52 (Step S30). When the TOP signal 36 has been input(YES in Step S30), the CPU 21 determines whether a BD signal 33 has beeninput to the first pixel size calculating portion 51 and the secondpixel size calculating portion 52 (Step S31). When the BD signal 33 hasbeen input (YES in Step S31), the first pixel size calculating portion51 and the second pixel size calculating portion 52 read the profilemagnification of the first pixel from the profile magnification holdingportion 25 (Step S32). The second pixel size calculating portion 52calculates the pixel size based on the profile magnification (Step S33).In other words, the second pixel size calculating portion 52 determinesto what size one pixel is enlarged, reduced, or maintained in terms ofthe magnification. The second pixel size calculating portion 52 outputsthe calculated pixel size to the profile magnification holding portion25. The CPU 21 determines whether output of one pixel corrected based onthe profile magnification has been completed (Step S34). When the outputof one pixel corrected based on the profile magnification has beencompleted (YES in Step S34), the CPU 21 determines whether output ofimage signals 31 corresponding to one line has been completed (StepS35). When the output of the image signals 31 corresponding to one linehas not been completed (NO in Step S35), the processing returns to StepS32. The CPU 21 similarly corrects the pixel size of the next pixelbased on the profile magnification. When the output of the image signals31 corresponding to one line has been completed (YES in Step S35), theCPU 21 determines whether the image formation has been completed (StepS41). When the image formation has not been completed (NO in Step S41),the processing returns to Step S31, and the image signals 31 for thenext line are generated.

Meanwhile, in parallel to the correction of the pixel size based on theprofile magnification by the second pixel size calculating portion 52described above, the first pixel size calculating portion 51 reads thecolor misregistration magnification from the color misregistrationmagnification holding portion 26 (Step S36). The first pixel sizecalculating portion 51 uses the magnification compositing portion 28 tocalculate the composite magnification of the profile magnification andthe color misregistration magnification with the use of Expression 4(Step S37). The first pixel size calculating portion 51 calculates thepixel size based on the image data, the composite magnification, and theerror amount at the time when the previous one pixel size is determined(Step S38). In other words, the first pixel size calculating portion 51determines to what size one pixel is enlarged, reduced, or maintained interms of magnification. The first pixel size calculating portion 51outputs the calculated pixel size to the image signal output portion 29.The image signal output portion 29 outputs, to the light source driveportion 18, the image signal 31 corresponding to one pixel based on thecalculated pixel size and the image data in the image data holdingportion 24 (Step S39). The light source drive portion 18 generates thedrive signal 35 based on the image signal 31, and outputs the drivesignal 35 to the light source 10. The light source 10 emits the lightbeam based on the drive signal 35 to form a latent image correspondingto one pixel on the photosensitive drum 708. The CPU 21 determineswhether output of image signals 31 corresponding to one line has beencompleted (Step S40). When the output of the image signals 31corresponding to one line has not been completed (NO in Step S40), theprocessing returns to Step S36. The CPU 21 similarly corrects a pixelsize of the next pixel. When the output of the image signals 31corresponding to one line has been completed (YES in Step S40), the CPU21 determines whether the image formation has been completed (Step S41).When the image formation has not been completed (NO in Step S41), theprocessing returns to Step S31, and image signals 31 for the next lineare generated. When the image formation has been completed (YES in StepS41), the CPU 21 ends the image formation.

FIG. 24 is a diagram for illustrating magnifications and pixel sizes incontinuous output operation of image signals 31. A pixel size of onepixel without magnification correction is 100 bit data. The colormisregistration magnification is +2%. The profile magnification is −25%for the first Block 0, and −20% for the next Block 1. The pixel size ofthe first pixel is changed, based on the profile magnification of −25%only, from 100 bit data to 75 bit data by the second pixel sizecalculating portion 52. In this example, an error of less than 1 bitdata is not generated by the correction based on the profilemagnification only, and hence the next pixel size is calculated withoutan error. As a result, timing at which the second pixel size calculatingportion 52 reads the profile magnification of the next pixel from theprofile magnification holding portion 25 and calculates the pixel sizeof the next pixel based on the profile magnification only is when timecorresponding to 75 bit data has elapsed.

In parallel to the calculation of the pixel size based on the profilemagnification only by the second pixel size calculating portion 52, thepixel size of the first pixel is determined as 77 bit data by the firstpixel size calculating portion 51 based on the composite magnificationof −23.5%. The pixel size is 76.5 bit data in calculation, but isdetermined as 77 bit data because the image signal 31 is output in unitsof 1 bit data. Therefore, an error of −0.5 bit data is generated. Theerror of −0.5 bit data is used when the pixel size of the next pixel isdetermined. As a result, timing at which the first pixel sizecalculating portion 51 reads the color misregistration magnification ofthe next pixel from the color misregistration magnification holdingportion 26 and determines the pixel size based on the compositemagnification is when time corresponding to 77 bit data has elapsed.

As described above, the timing at which the pixel size is calculatedbased on the composite magnification is displaced from the timing atwhich the pixel size is calculated based on the profile magnificationonly. As illustrated at around the center of FIG. 24, the profilemagnification is changed when the block transitions from Block 0 toBlock 1. In this example, 76 bit data at the time when the blocktransitions from Block 0 to Block 1 is calculated based on the compositemagnification of −23.5%. However, of 76 bit data, 12 bit data in Block 1is to be determined based on the composite magnification of −18.4%. Toaddress this problem, the first pixel size calculating portion 51 uses adisplacement amount of the calculation timing of the pixel size as anerror amount in the next calculation of the pixel size. The last pixelsize of Block 0 illustrated in FIG. 24 is calculated as 76 bit data withthe use of 76.5 bit data determined from the composite magnification of−23.5% and the error of −0.5 bit data from the previous pixel. It isdetermined that there is no error amount for use in calculation of thepixel size of the next pixel, and the block of the profile magnificationis switched from Block 0 to Block 1. Based on the difference between thetiming at which the pixel size is calculated based on the compositemagnification (first timing) and the timing at which the pixel size iscalculated based on the profile magnification only (second timing), only12 bit data of 76 bit data is required to be corrected. 12 bit data isexcessively reduced by 5.1% (=−18.4%−(−23.5%)) corresponding to thechange of the composite magnification. In order to correct the excessivereduction, 0.6 bit data (=12 bit data×5.1%) is used as the error amountwhen the pixel size of the next pixel is calculated.

According to the second embodiment, the difference (displacement amount)between the first timing at which the pixel size is determined based onthe profile magnification and the second timing at which the pixel sizeis determined based on the composite magnification can be used tocorrect the image data based on the composite magnification. Therefore,as in the first embodiment, the image with the reduced colormisregistration can be formed.

According to the second embodiment, there can be reduced the colormisregistration at the time when the profile magnification (firstmagnification) and the color misregistration magnification (secondmagnification) of the image in the main scanning direction are correctedin combination.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2017-068629, filed Mar. 30, 2017, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An image forming apparatus, comprising: an imageforming portion including: (i) a first photosensitive member on which atoner image of a reference color for color misregistration correction isformed; (ii) a plurality of photosensitive members on which toner imagesof different colors from the first photosensitive member are formed; and(iii) a scanning unit having a plurality of light sources configured toemit light beams based on image signals, the scanning unit beingconfigured to scan the light beams, emitted from the plurality of lightsources, on respective photosensitive members, wherein the image formingportion is configured to form color misregistration measurement tonerpatterns for measuring displacement amounts of other colors with respectto the reference color; an optical sensor configured to detect the colormisregistration measurement toner patterns; a storage portion configuredto store profile data of an image with respect to a scanning position ina main scanning direction, the profile data including data which isgenerated based on a profile representing change in a scanning speed ofthe light beam scanning the photosensitive member with respect to eachof a plurality of scanning positions in the main scanning direction andwhich corresponds to each of the plurality of scanning positions in themain scanning direction, the profile data being provided for each ofcolors; and a controller configured to: (i) execute a firstmagnification correction process, based on a detection result of thecolor misregistration measurement toner pattern, on image data of theother colors than the reference color; (ii) execute a secondmagnification correction process, based on the profile data, on imagedata of the reference color and image data corrected in the firstmagnification correction process; and (iii) control the plurality oflight sources corresponding to the image data, respectively, based onimage data corrected in the second magnification correction process. 2.An image forming apparatus according to claim 1, wherein the profiledata with respect to the scanning position is a profile indicating amagnification of each of a plurality of areas in the main scanningdirection.
 3. An image forming apparatus, comprising: an image formingportion including: (i) a first photosensitive member on which a tonerimage of a reference color for color misregistration correction isformed; (ii) a plurality of photosensitive members on which toner imagesof different colors from the first photosensitive member are formed; and(iii) a scanning unit having a plurality of light sources configured toemit light beams based on image signals, the scanning unit beingconfigured to scan the light beams, emitted from the plurality of lightsources, on respective photosensitive members, wherein the image formingportion is configured to form color misregistration measurement tonerpatterns for measuring displacement amounts of other colors with respectto the reference color; an optical sensor configured to detect the colormisregistration measurement toner patterns; a storage portion configuredto store profile data of an image with respect to a scanning position ina main scanning direction, the profile data including data which isgenerated based on a profile representing change in a scanning speed ofthe light beam scanning the photosensitive member with respect to eachof a plurality of scanning positions in the main scanning direction andwhich corresponds to each of the plurality of scanning positions in themain scanning direction, the profile data being provided for each ofcolors; and a controller configured to: (i) execute a firstmagnification correction process, based on a detection result of thecolor misregistration measurement toner pattern, on image data of theother colors than the reference color; (ii) execute a secondmagnification correction process, based on the profile data, on imagedata of the reference color and image data corrected in the firstmagnification correction process; and (iii) control the plurality oflight sources corresponding to the image data, respectively, based onimage data corrected in the second magnification correction process, andbased on a difference between a first timing at which the image signalis corrected in the first magnification correction process and a secondtiming at which the plurality of light sources are controlled by thecontroller.
 4. An image forming apparatus according to claim 3, whereinprofile data with respect to the scanning position is a profileindicating a magnification of each of a plurality of areas in the mainscanning direction.